Work machines such as, for example, wheel loaders, track type tractors, and other types of machinery are used for a variety of tasks. These work machines may include a power source, such as a diesel engine, a gasoline engine, a natural gas engine, or any other type of power source, that provides the power required to complete these tasks. In certain power systems, the power source may be coupled to a generator to produce an electrical power output supplied to one or more electric motors. The motors may be connected to ground engaging traction devices to propel the work machine.
The electric motors coupled to the traction devices may include, for example, AC induction motors. While these types of motors may be capable of operating for brief periods at peak torque levels greater than their continuous rating, the high currents associated with these peak torque levels can lead to damage of various electric motor components. For example, a sustained, high-current condition (i.e., an overload condition) may result in elevated temperatures in the conductive windings of the motor stator. These elevated temperatures can damage the insulation of the windings, which can lead to eventual or even immediate conductor failure.
The electric motors may be protected from potential damage by ensuring that the temperature of the conductive windings of the stator does not exceed a desired level. Monitoring this temperature, however, can be challenging. Particularly, temperature sensing devices, even if placed directly adjacent to the windings, can provide inaccurate measurements of the winding temperature. Due to the large mass of metal that may be used to form the motor stator in, especially, heavy duty electric motors for high power traction applications, the motors may have large thermal time constants. Thus, in response to high currents in the stator conductors, the temperature of the conductors can significantly exceed a specified rating before the surrounding areas, including the temperature sensors, exhibit even a modest temperature change. Thus, in order to protect an electric motor from damage caused by overload conditions, a system for accurately determining the temperature of the stator windings of an electric motor may be required.
At least one system has been developed for protecting against an overload in an electric motor. For example, U.S. Pat. No. 5,510,687 (“the '687 patent), issued to Unsworth et al. on Apr. 23, 1996, describes a system for detecting an overload condition in an electric motor by estimating the temperature of the motor. Particularly, the system of the '687 patent introduces a DC voltage component onto a stator winding of the electric motor. The system includes circuitry to determine the DC voltage drop across the winding. The system also includes a Hall effect device for sensing the current in the winding attributed to the DC voltage. Based on the sensed voltage and current values, the resistance of the winding may be calculated using Ohm's law. The temperature of the winding may be predicted by comparing the calculated resistance value to a calibrated resistance value determined for a known temperature. If the predicted temperature exceeds a predetermined threshold value, the electric motor is shut down.
While the system of the '687 patent may be effective for avoiding overload conditions in certain situations, the system of the '687 patent includes several disadvantages. For example, the system includes complex circuitry for measuring the DC voltage and current signals in the motor winding. This circuitry can add expense to the motor and may adversely affect its reliability. Further, the system must actively compensate for the effects on motor operation caused by the introduced DC voltage component. Because each motor may exhibit unique resistance characteristics, each motor may require calibration prior to operation to determine a baseline resistance value at a known temperature. This can result in added manufacturing expense. Also, upon detection of an overload condition, the system of the '687 patent fully shuts down the motor rather than allowing the motor to continue operating with a reduced output level.
The present disclosure improves upon the prior art systems for protecting against an overload condition in an electric motor.